An MD apparatus is one example of conventional digital audio devices capable of recording and reproducing audio signals. In such a digital audio device, when listening to audio signals of, for example, music, by outputting the audio signals as monitor outputs at the same time the audio signals are recorded, the following operation is performed. The input rate of digital audio signals to be input varies. The digital signals input at the varying input rate are first guided to a digital PLL (phase locked loop). In the digital PLL circuit, clocks corresponding to the sampling frequency of the input digital audio signals, for example, 48 KHz, 44.1 KHz or 32 KHz, are extracted. Signals, such as data synchronous with the clocks extracted by the PLL circuit, bit clock, LR signal, and D/A converter-use master clock, are input to a D/A converter in the next stage, for converting these signals into analog signals. Then, the monitor outputs of analog signals are obtained.
However, in the conventional digital audio device, when digital audio signals to be recorded on a recording medium are supplied as digital optical inputs, noise is produced due to external interference or a variation in the input rate of digital audio signals. As a result, a fluctuation occurs in the clocks output by the digital PLL circuit. The fluctuation in the clocks causes noise in the D/A converter for converting a digital signal into an analog signal in the next stage, and disturbs the audio signals of music, etc., resulting in undesired sounds.